Abstract: An intensity control for fluorescent lamps of the type wherein a triac in series with the lamp provides control of the lamp current. Triac gating is controlled by a novel phase-lock circuit. In the phase-lock circuit, voltage squaring means are provided for producing unipolar square waves having a pulse width equal to a corresponding half cycle of the a.c. input power. Ramp waves generated from the square waves are applied to the base of a PNP transistor. Current through the lamp is sampled and produces a proportional voltage signal which is superimposed on a parallel path of the square waves to create a phase reference signal. An intensity control potentiometer supplies a selectable d.c. intensity control signal of polarity opposite the phase reference signal, and the intensity control signal and phase reference signal are algebraically added together. The resultant signals apply to the inverting input of integrating amplifier which reverses the polarity and integrates the resultant signal.